1. Field of the Invention
The present invention relates to a speaker and a speaker diaphragm, and more particularly to a speaker of small diameter and a speaker diaphragm.
2. Description of the Related Art
Conventionally, a speaker of small diameter has been employed in an audio system for vehicle because of a limited installation space.
The reproduction limit of low sound in the speaker depends on a low sound resonance frequency f0 as a rule of thumb, and this value is represented by the following expression (1),f0=(½π)(s0/m0)1/2  (1)
where m0(gr) is an effective mass of a vibration system, and s0(dyne/cm) is a stiffness (flexural rigidity (hardness) for enabling less vibration of a diaphragm main body) of a support portion for the vibration system. Accordingly, in order to decrease the reproduction limit of low sound, the stiffness s0 is reduced by making flexible an edge portion, which resiliently supports the diaphragm main body on the frame, or the weight of a diaphragm or a voice coil is increased to make the effective mass m0 greater.
For the speaker of small diameter, it is requisite to lighten the weight of the diaphragm to miniaturize a coil or a magnetic circuit for driving the diaphragm, whereas it is less negligible to enhance the reproducing capacity of low sound range to aim at the high quality reproduction.
In designing the speaker of small diameter, if the effective mass m0 of the vibration system is decreased for lighter weight, the stiffness s0 must be reduced by an excess amount. Thereby, to sufficiently reduce the stiffness s0, it is required to contrive the structure of the edge portion provided around the outer circumference of the diaphragm main body to resiliently support the diaphragm main body on the frame or the linkage structure between the edge portion and the frame.
From such a background, the speaker for use in the audio system for vehicle has hitherto employed a thin film for the diaphragm to lighten the weight. Furthermore, the edge portion 1 provided around the outer circumference of the diaphragm main body to resiliently support the diaphragm main body on the frame has a plurality of groove ribs 3 integrally molded to enhance the reproducing capacity of low sound range, as shown in FIG. 7.
This rib 3 is a groove having a V-character shape in transverse section, so-called a tangential edge, which extends in a tangential direction of the inner circumferential edge 1a (i.e., outer circumferential edge of the diaphragm main body) of the edge portion 1 and is disposed at an equal interval in a circumferential direction. This rib 3 increases the flexural rigidity in an extending direction of the groove (i.e., a tangential direction of the outer circumferential edge of the diaphragm main body), but serves to decrease the flexural rigidity due to an expansion or contraction of the groove width in a crossing direction of the groove (i.e., a radial direction of the diaphragm main body). Consequently, the rib 3 makes the diaphragm easy to move and reduces the stiffness s0.
However, if the stiffness so is reduced excessively, the diaphragm main body causes the reproduced sound to be distorted, or gives rise to a rolling-that is a factor of causing the noise due to the mutual contact of a vibration mechanism, resulting in the risk of degrading the quality of reproduced sound. Thereby, there was the problem that the reproducing capacity of low sound range could not be enhanced only by reducing the stiffness s0.
Further, an improved method of low sound range by forming the rib 3 had a drawback that since the rib 3 is intermittently disposed, the reinforcement or easiness of deformation of the rib 3 can not be attained uniformly over the entire area of the edge portion 1, and the edge portion 1 is fixed to the frame less uniformly over the entire circumference, whereby there is the possibility that the load on the edge portion 1 becomes maximum in a specific region due to a vibration propagation at the time of reproducing the sound. Due to the above reasons, when an input signal was large (large input), or the diaphragm was made thinner, there was the risk that the quality of reproduced sound was degraded due to the rolling of the edge portion or the local distortion. Also, there was the risk that the rolling might occur when the weight or compliance was unbalanced to lay a voice coil lead wire on the edge portion 1.